One of the conventional methods for producing an electron tube is disclosed by Japanese Patent Application Laid-Open No. 59-94325. FIG. 1 illustrates a structure of electron tube, wherein 1 is a shadow mask, 1a is the shadow mask side facing an electron gun, 2 is an electron gun, 3 is a fluorescent plane, 4 are electron beams, and 5 is an electron tube. The shadow mask made of a metallic material is provided with a number of openings, and is designed to match with the fluorescent plane. When the electron tube is switched on, the electron beams issued by the electron gun pass through the beam-transmitting openings to hit the fluorescent plane, generating desired images thereon.
Most of the electrons, however, hit the shadow mask without passing the openings, with the result that energy of motion of the electrons is transmitted as thermal energy to the shadow mask, to heat it to 70.degree. C. or higher. This temperature rise thermally expands the shadow mask to cause misalignment between the opening sections in the shadow mask and fluorescent plane, causing problems, e.g., color shift and lowered brightness. The problematic phenomenon of thermal expansion of the shadow mask caused by shooting electron beams is referred to as doming.
It is known that these problems are controlled by coating the shadow mask side 1a facing the electron gun with a coating material containing an element having an atomic number of 70 or more to form an electron beam reflecting film. In particular, the coating material containing powdered bismuth oxide or the like is considered to be suitable, because of its effect of efficiently reflecting the electron beams (hereinafter referred to as electron-reflecting effect). It is also known that an element having a larger atomic number shows a larger electron-reflecting effect. Therefore, the shadow mask side 1a is coated with a coating material containing a material of large electron-reflecting effect, such as powdered bismuth oxide, to reflect the electron beams that hit the shadow mask. The electron beam reflecting film is generally formed by spraying, in which the electron beam reflecting coating material is supplied by a high-capacity pump, e.g., magnet pump, to a nozzle to prevent settling of the coating material in the nozzle and piping systems, and the nozzle is scanned over the shadow mask to form the film. The electron beam reflecting film formed on the shadow mask side facing the electron gun prevents temperature rise of the shadow mask, and thereby to solve the problems, such as color shift, caused by doming.
The surface coat is formed by spreading a surface coating material, such as that containing SiO.sub.2 or ITO, to realize a low-reflection function and anti-static function by resultant difference in refractive index and its conductivity. The coating material is spread by spraying or spin coating, the latter being a normal choice, because of difficulty of the former to give a dense, homogeneous coating film.
A common method for producing the coating material for electron beam reflecting films is dispersion by a rotating device, such as a ball mill. However, the coating material dispersed by this method tends to suffer secondary agglomeration, after it is dispersion-treated, which causes problems, e.g., settlement of the coating material in, and clogging of, the coating systems, making it difficult to inject the coating material stably from the nozzle and to form a dense, homogeneous electron beam reflecting film. Another dispersion method uses a sand mill. The method using such a medium, however, has disadvantages, e.g., breakdown of the medium itself in a dispersion machine to contaminate the coating material, and unstable dispersion conditions of the coating material, because of newly evolved interfaces as a result of destruction of coating material particle shapes.
Moreover, the conventional coating material for electron beam reflecting films contains particles of large average size and unstable particle size distribution. In order to secure a high surface coverage of the electron beam reflecting film, it is necessary to spread a large quantity of the coating material over the shadow mask side, to 0.2 mg/cm.sup.2 or more, as disclosed by Japanese Patent Application Laid-Open No. 59-94325. As a result, the film tends to come off from the shadow mask in the electron tube product, causing problems, e.g., contamination within the electron tube and lowered image quality.
The method for forming an electron beam reflecting film by spraying supplies the coating material to the nozzle and recycles it by a high-capacity pump, e.g., a magnet pump. This method, however, involves problems, e.g., adverse effects of fluctuating pump discharge pressure on discharge conditions of the coating material at the nozzle, causing uneven coating as a result of fluctuations in quantities discharged from the nozzle and making it difficult to form a dense, homogeneous electron beam reflecting film. Moreover, a head (level difference) between the surface of the stored coating material and the nozzle changes as the coating material is spread over the shadow mask. This causes a change in pressure for supplying the coating material to the nozzle and therefore a change in quantity of the coating material discharged from the nozzle. This also causes uneven coating and makes it difficult to form a dense, homogeneous electron beam reflecting film.
Denser coating is needed for forming a surface coat over a glass panel surface by spraying, so that the surface coat can exhibit a low-reflection function and anti-static function. The conventional spraying method, however, tends to form an uneven film, and difficult to realize the surface coat exhibiting sufficient functions. The spin coating for surface coat has disadvantages such as low coating efficiency and high cost.